1. Field of the Invention
The present invention provides a method and related apparatus for calibrating a tilt servo system of an optical disk drive, and more particularly, a tilt servo system calibration method and related apparatus for surface bend according to a physical model showing that the optical disk has different degrees of radial tilt at different locations.
2. Description of the Prior Art
In modern information society, small, light, high-density, and low-cost optical disks have become one of the most popular non-volatile storage media. In order to access high-density optical data in an optical disk, the key development issue is how to improve precise operations of an optical disk storage device (such as optical disk drives and CD players).
Please refer to FIG. 1 and FIG. 2. FIG. 1 illustrates a block diagram of a prior art optical disk drive 10, while FIG. 2 is a lateral view diagram of the optical disk drive 10 along a sectioning-line 2—2. The optical disk drive 10 includes a control module 20, a tilt servo system 22, and related mechanical and electrical structures for data access, such as a motor 16, a track 14, a sled 12A, and a pick-up head 12B. The control module 20 controls operation of the optical disk drive 10; the motor 16 rotates an optical disk 18. The sled 12A slides along the track 14. The pick-up head 12B set on the sled 12A emits a laser beam to the optical disk 18, and can access data at different locations on the optical disk 18.
As those skilled in the art recognize, the laser beam from the pick-up head 12B should be accurately focused on the optical disk 18 for data access (including reading data from the optical disk, or writing/burning data to the optical disk). Therefore, after focusing the laser beam on a surface 24 of the optical disk 18 (see FIG. 2), the pick-up head 12B detects a laser reflection from the optical disk 18 then generates a corresponding signal which is returned to the control module 20. The control module 20 determines whether the pick-up head 12B has already focused the laser beam accurately on the optical disk 18 according to the received reflection. If the pick-up head 12B has not focused accurately, the control module 20 can control a servomechanism on the sled 12A with a servo signal Fp to fine tune the location of the pick-up head 12B up and down (along an arrow 26 in FIG. 2), so as to adjust the distance between the pick-up head 12B and the optical disk 18. After the distance between the pick-up head 12B and the surface of the optical disk 18 is adjusted, the focusing condition between the pick-up head 12B and the optical disk 18 changes accordingly, and so does the reflection received by the pick-up head 12B. Again, according to the received reflection, the control module 20 adjusts/calibrates the distance between the pick-up head 12B and the optical disk 18 with the servo signal Fp. Repeating the above focusing feedback control loop, the pick-up head 12B can finally focus on the optical disk 18 accurately. Usually, there is a fixed relationship between values of the servo signal Fp and the vertical position of the pick-up head (along the arrow 26). For example, as FIG. 2 illustrates, if the value of the servo signal Fp is equal to a standard servo signal Fs0, the pick-up head 12B can focus exactly on the surface 24 of the optical disk 18, and if the servo signal Fp is greater than the standard servo signal Fs0, the sled 12A is driven to raise the pick-up head 12B by the servo signal Fp; that is, if a location of the surface 24 is fixed, the corresponding servo signal Fp triggers the sled 12A to maintain, increase, or decrease the distance between the pick-up head 12B and the surface 24 based on whether the servo signal Fp is equal to, greater, or smaller than the servo signal Fs0. In other words, the value of the servo signal Fp corresponds to the vertical position of the pick-up head 12B.
In general, the optical disk drive 10 includes a tray or equivalent mechanism to bear the optical disk 18. In a preferred situation, the surface of the optical disk 18 should remain parallel to the pick-up head 12B for steady data access. However, in practice, owing either to blemishing of the tray, or surface bend of the optical disk 18 (or any other inferior qualities in the optical disk drive 10), an angle commonly exists between the surface of the optical disk 18 and the horizontal plane in which the pick-up head 12B moves. In this case, the optical disk drive 10 further includes a tilt servo system 22, which determines how to adjust the angle between the surface 24 and the pick-up head 12B according to the servo signal Fp provided by the control module 20. As to operations of the tilt servo system 22, please refer to FIG. 3, FIG. 4 (also FIG. 1 and FIG. 2).
FIG. 2, FIG. 3 and FIG. 4 are also lateral view diagrams of the optical disk drive 10. In FIG. 2, if the surface 24 of the optical disk 18 parallels the pick-up head 12B, a distance D between the sled 12A and the surface 24 is exactly equal to the distance D0, hence the pick-up head 12B focuses on the surface 24 accurately. FIG. 2 illustrates a horizontal projection 28 with a dotted line, which is in a plane parallel to the pick-up head 12B. As FIG. 3 illustrates, a non-zero angle exists between the surface 24 and the horizontal plane of the pick-up head 12B, i.e. the surface 24 is no longer perpendicular with respect to the focus (vertical) axis of the pick-up head. Therefore, the distance between the surface 24 and the sled 12A deviates from the distance D0 to the upper side of the horizontal 28 (as shown in FIG. 3, the disk 18 has effectively risen upward, so the distance D is greater than the distance D0). Because the surface 24 rises, the pick-up head 12B cannot focus on the optical disk accurately, and so, referring again to FIG. 3, the focusing feedback control mechanism between the pick-up head 12B and the control module 20 takes effect. The control module 20 drives the servomechanism of the sled 12A with a servo signal Fp greater than the standard servo signal Fs0, so that the pick-up head 12B rises along an arrow 27A in compensation, to the end that the pick-up head 12B can focus accurately on the surface 24 once more.
Moreover, the tilt servo system 22 also monitors the servo signal Fp provided by the control module 20. If the servo signal Fp is greater than the standard servo signal Fs0, the tilt servo system 22 determines the surface 24 has already deviated from the horizontal 28, because the control module 20 has increased the servo signal Fp in order to re-focus the pick-up head 12B on the surface 24. In this situation, the tilt servo system 22 starts adjusting the angle between the optical disk 18 and the pick-up head 12B. As FIG. 4 illustrates, in accordance with the servo signal Fp, the tilt servo system 22 lowers the optical disk 18 along the arrow 27C, which effectively lowers the surface 24 toward its original vertical position above the pick-up head 12B. While the tilt servo system 22 adjusts the tilt of the optical disk 18, the focusing feedback control mechanism between the control module 20 and the pick-up head 12B decreases the servo signal Fp gradually until the surface 24 returns to the horizontal 28. As a result, the distance D between the surface 24 and the sled 12A returns to a value equal to the distance D0, and the servo signal Fp from a value greater than the standard servo signal Fs0 in FIG. 3, descends to a level almost equal to the standard servo signal Fs0. At the same time, by monitoring the servo signal Fp continually, the tilt servo system 22 determines whether tilt servo operation has been completed, that is, whether the surface 24 is once more coincident to the horizontal 28. Hence, the tilt servo system 22 finishes adjusting tilt angle. In conclusion, the tilt servo system 22 adjusts the tilt angle of the tray with the servomechanism, or equivalently changes the vertical position relationship between the optical disk 18 (and hence the surface 24) and the pick-up head 12B.
From the above-mentioned descriptions, during accessing data stored on the optical disk 18, according to the difference between the servo signal Fp and the standard servo signal Fs0, the tilt servo system 22 adjusts the distance between the optical disk 18 and the pick-up head 12B until the servo signal Fp is equal to the standard servo signal Fs0. Because operations of the tilt servo system 22 are based on the standard servo signal Fs0, measurement and calibration of the standard servo signal Fs0 becomes a key point.
In order to set the standard servo signal Fs0 for the tilt servo system 22, the optical disk drive 10 goes through special steps before the tilt servo system 22 is enabled. Please refer to FIG. 5 (also FIG. 1 and FIG. 2) illustrating a prior art process 100, which shows a process of the optical disk drive 10 in FIG. 1 calibrating the tilt servo system 22. Process 100 includes the following steps:                step 102: start calibration of the tilt servo system 22. (Process 100 includes calibrating and setting the standard servo signal Fs0 for the tilt servo system 22 before data is accessed, but after a disk is inserted into the optical disk drive 10. As mentioned above, both blemishes/irregularities of the optical disk drive (or other support devices for the optical disk 18) and surface bend of the optical disk 18 cause tilt and vertical deviation of the surface 24, so that when inserting a disk, the standard servo signal Fs0 of the tilt servo system 22 may be in error and should ideally be calibrated after the tilt servo).        step 104: the sled 12A carrying the pick-up head 12B seeks two different locations, P0 and P1, along the track 14 (please refer to FIG. 2), where the focusing feedback control mechanism between the pick-up head 12B and the control module 20 is engaged. As mentioned above, the focusing feedback control mechanism adjusts value of the servo signal Fp, so as to move the pick-up head 12B up and down for accurate focus on the surface 24. Additionally, owing to the fixed focus point of the pick-up head 12B, the vertical position of the pick-up head 12B will correspond to any undulation of the surface 24 and the servo signal Fp should change accordingly. Therefore, the value of the servo signal Fp at the locations P0 and P1 represent any undulation of the surface 24 (or deviation from the horizontal). Therefore, the greater the difference in the servo signal Fp between the locations P0 and P1, the more deviation from the horizontal there is in the optical disk 18. Taking the values of the servo signal Fp at the locations P0 and P1, the prior art process 100 calibrates tilt of the surface 24.        step 106: according to an assumption that a surface is a perfect plane, process 100 estimates the overall tilt angle of the surface 24 away from the horizontal 28 based on the difference in vertical height between the locations P0 and P1. After that, process 100 determines a compensating amount required for the surface 24 to be returned to the horizontal 28.        step 108: according to the compensating amount in step 106, the tilt servo system 22 adjusts the optical disk 18, so as to return the surface 24 to the horizontal 28.        step 110: the sled 12A carrying the pick-up head 12B seeks the location P0 again and the focusing feedback control mechanism acts to find focus. The resulting value of the servo signal Fp corresponding to the location P0 is set as the standard servo signal Fs0. Therefore, the process 100 finishes setting the standard servo signal Fs0. As FIG. 2, FIG. 3, and FIG. 4 illustrate, the servo signal Fp should be equal to the standard servo signal Fs0 if the surface 24 is horizontal. Furthermore, the function of the tilt servo system 22 is to maintain a minimum differential between the servo signal Fp and the standard servo signal Fs0, or equivalently to maintain the disposition of the surface 24 in the horizontal plane. Since the prior art process 100 calibrates and adjusts the tilt angle under the assumption that optical disks are perfect planes, the surface 24 should be horizontal after step 108 where the servo signal Fp is taken as the standard servo signal Fs0.        step 112: after setting the standard servo signal Fs0, the tilt servo system 22 acts with reference to the standard servo signal Fs0.        step 114: finish process 100. The optical disk drive 10 starts accessing the optical disk 18, while the tilt servo system 22 continues to adjust tilt angle, so that the servo signal Fp is always close to the standard servo signal Fs0 set in step 110.        
To further illustrate the prior art process 100, please refer to FIG. 6, FIG. 7, and FIG. 8, which continue on from FIG. 1 to FIG. 4. As mentioned above, owing to blemishes/irregularity of the mechanism or the surface, the surface 24 of the optical disk 18 is not parallel to the horizontal 28, which means the optical drive 10 should undergo process 100 first. Initially, as FIG. 6 illustrates, the sled 12A seeks the location P0 where the servo signal Fp provided by the focusing feedback control mechanism is hypothetically equal to a signal f0. Moreover, the distance between the surface 24 and the horizontal 28 at the location P0 is hypothetically equal to a distance Yp0 (or equivalently a distance between the surface 24 and the sled 12A). Then, in FIG. 7, the sled 12A moves to another location P1, a distance DX0 apart from the location P0, where the servo signal Fp is hypothetically equal to a signal f1, which corresponds to the distance between the surface 24 and the sled 12A at the location P1 (or a distance Yp1). As FIG. 6 and FIG. 7 illustrate, since the distance Yp1 is greater than the distance Yp0, the control module 20 applies a greater servo signal Fp to the pick-up head 12B in the location P1 for accurate focus. In brief, the difference in servo signal Fp between the locations P1 and P0 (also the signals f1 and f0) corresponds to the difference in vertical height of the surface 24 between the locations P1 and P0 (also the difference between distances Yp1 and Yp0).
After calibrating tilt angle in step 104, process 100 estimates overall tilt angle of the surface 24 based on the assumption that the surface is a perfect plane in, step 106. As the inset graph 7A in FIG. 7 illustrates, since the distance between the locations P0 and P1 is the distance DX0, and difference in vertical height of the surface 18 between the locations P0 and P1 is a distance DY0 (=Yp1−Yp0), an angle A1 between the surface 24 and the horizontal 28 can be calculated. Under the assumption that the surface 24 is a perfect plane, the tilt servo system 22 compensates according to the angle A1 in step 108. Continuing on from FIG. 6 and FIG. 7, in FIG. 8, the tilt servo system 22 compensates according to the angle A1 along the arrow 27C in step 108 (wherein a dotted line 25 represents the position of the optical disk 18 in FIG. 6 and FIG. 7). Therefore, if the surface 24 of the optical disk 18 is indeed a perfect plane, the surface 24 should be parallel to the horizontal 28 after compensation. Then, in step 110, the sled 12A returns to the location P0, while the current servo signal Fp provided by the focusing feedback control mechanism is set for the standard servo signal Fs0. As a result, the tilt servo system 22 starts to act (step 112), and finishes calibration (step 114), so as to facilitate accessing of the optical disk 18.
As mentioned above, the calibration provided by the tilt servo system in the prior art process 100 is carried out under the assumption that the surface of the optical disk is a perfect plane. However, in practice, the surfaces of optical disks are not perfect planes, but may be subject to slight bending or ‘dishing’, the extent of which cannot be seen by the naked eye (in the order of micrometers). In the process of high-precision data access, optical disk bend can cause many negative effects. More especially in that the presence of bending undermines the assumption that the surface is a perfect plane, the prior art process 100 fails to calibrate the correct standard servo signal, to the end that the tilt servo system cannot access data properly. Please refer to FIG. 9 and FIG. 10 for details. Continuing from FIG. 2 to FIG. 4 and FIG. 6 to FIG. 8, FIG. 9 and FIG. 10 are also lateral view diagrams.
FIG. 9 and FIG. 10 show exaggerated bending of the surface 24 for convenience. It can be seen that different locations on the surface may have varying degrees of tilt. As FIG. 9 illustrates, a point P is a hypothetical fulcrum when the tilt servo system 22 calibrates the surface 24. Additionally, vertical lines through the locations P0 and P1 intersect the surface 24 at points Pa0 and Pa1, and a horizontal line projected from the point Pa0 intersects the vertical line passing through the point Pa1 at a point Pa01. Furthermore, an included angle exists between a line from the point Pa0 to the point P and the horizontal 28, while another included angle A1 exists between a line from the point Pa1 to the point Pa0 and a line from the point Pa0 to the point Pa01 (see FIG. 9 or its inset graph 9A). Because the surface 24 is not a perfect plane, the angle A is not equal to the angle A1. As a result, in step 106, the prior art process 100 can only estimate the angle A1 with the distances DX0 and DY0, but not the angle A. That is, the prior art process 100 can only estimate the included angle of surface bend between two points and cannot compensate for variations in included angle across the wider data area of the optical disk 18.
FIG. 10 (and its inset graph 10A) illustrates calibration error when adjusting for surface bend in step 108. As the prior art process 100 compensates according to the angle A1 along the arrow 27C in step 108, only the line from the point Pa0 to the point Pa1 is parallel to the horizontal 28, while an included angle (A-A1) still exists between the line from the point P to the point Pa0 and the horizontal 28. In other words, when the pick-up head 12B again calibrates the servo signal Fp at the location P0 in step 110, the surface 24 still deviates from the horizontal 28. Therefore, as FIG. 10 illustrates, a calibration error occurs, which means that the surface 24 near the location P0 (or near the point Pa0) is not horizontal. If, in subsequent operations, the tilt servo system 22 always takes this standard servo signal Fs0 as a reference, the tilt servo system 22 will not operate properly and neither will the focusing feedback control mechanism.
In summary, since the prior art process 100 in FIG. 5 is based on the assumption that the surface is a perfect plane, the prior art technique relies on readings from only two locations for the whole surface tilt estimation. However, the surfaces of optical disks are generally bent or dished to some degree and in such a way that their surfaces have different degrees of tilt at different locations. That is, the prior art process 100 cannot derive a correct standard servo signal, with the result that the tilt servo system 22 cannot adjust the tilt surface precisely. Consequently, the optical disk drive 10 may suffer from degradation of data read-back signals.